This invention relates to an antenna arrangement for transducing signals broadcast in a cellular mobile radio communications system, and in particular but not exclusively to such an arrangement for use in the uplink (mobile transmit to base station receive) of a cellular mobile radio communications system.
In a cellular mobile radio communications system, a number of base stations are disposed over an area notionally divided into a number of cells, each base station being associated with a cell. Each cell is assigned a number of radio frequency channels on which mobile units within the cell transmit and receive signals to and from the cell""s base station. The frequency channels assigned to adjacent cells are generally different to permit frequency re-use.
Each mobile unit when in use may transmit a signal to the base station on an uplink of the communication system. The strength of the uplink signal received at the base station will vary according to the location of the mobile unit within the cell, and the received signal may be degraded as a result of multipath effects and a low signal to noise ratio on the uplink. One way to improve the received signal to noise ratio on the uplink is to improve the uplink receiving antenna diversity gain.
In environments such as rural environments the uplink receiving antenna gain remains an undesirable constraint on the cell size. In rural areas, only rarely if ever will the full capacity of frequency channels assigned to a cell be simultaneously in use, whilst there are unwanted gaps in the coverage of the available cells when the base stations are widely dispersed.
There are known various arrangements for improving the uplink gain of the base station receiving antenna array in a cellular mobile radio system. Second generation cellular systems commonly use two spaced antenna arrays in each base station sector for receiving the uplink signal with space-diversity. The signals received at the two spatially separated antenna arrays will have travelled different spatial paths from the mobile unit and hence will each arrive with different fading envelopes, and the signal received at one antenna array may be used to augment the signal received at the other. The antenna array outputs are combined to improve the uplink receiving antenna system gain.
To further improve uplink receiving antenna gain other that of the above arrangement, it has been proposed to use a four-branch space-diversity antenna arrangement. Four vertical polarization antenna arrays are spaced apart at the base station. Each of the four outputs from the spaced antenna arrays are combined to provide a higher space-diversity gain than that provided by the two-branch space-diversity antenna arrangement.
These space-diversity antenna arrangements utilise antennas receiving only the vertical polarization components of the uplink signal.
In another known uplink receiving antenna arrangement for a cellular mobile communications system, gain is improved by the use of polarization diversity at the base station. The vertically polarized signal component is received on one antenna array and a horizontally polarized signal component signal is received on a different antenna array. The two outputs are fed into a combiner to produce an output with a polarization diversity gain.
In accordance with the present invention there is provided antenna apparatus for receiving uplink signals from mobile units in a cellular mobile radio communications system, said arrangement comprising:
first antenna means for receiving an uplink signal at a first polarization;
second antenna means for receiving the uplink signal at a second polarization different to said first polarization;
third antenna means for receiving the uplink signal at a third polarization at a location spaced from said first antenna means by a distance sufficient for achieving space-diversity with respect to said first antenna means;
fourth antenna means for receiving the uplink signal at a fourth polarization, different to said third polarization, at a location spaced from said second antenna means by a distance sufficient for achieving space-diversity with respect to said second antenna means; and
means for combining the uplink signal received at each of said antenna means to generate a combined received signal,
wherein said second antenna means is located closer to said first antenna means than to said third antenna means, and
said fourth antenna means is located closer to said third antenna means than to said second antenna means.
An uplink signal in a cellular mobile communications system will often have both vertical polarization and horizontal polarization components. For example, where the mobile unit is a hand-held mobile unit with an azimuthally inclined omni-directional antenna, the signal radiated from the mobile unit will in most directions have vertical and horizontal polarization components which vary according to the inclination at which the mobile unit is held. Consequently, both of these orthogonally-resolved signal components are generally received at a base station. Thus, with the present invention, it is possible to improve the diversity gain of the uplink receiving antenna, to achieve a gain comparable to that of the four-branch space-diversity arrangement.
Such an uplink diversity gain improvement at a base station of a cellular mobile radio communications system will effectively apply a radio link budget gain which serves to improve the operational range of the radio cells of the system. The radio link budget determines the levels of in-building coverage and street level coverage in a cell. An increase in cell range such as by enhanced gain in the uplink and higher transmit power in the downlink has a fundamental effect on the number of base station systems required in a system, and hence also the associated cost.
The visual size of an aerial at a base station, whether situated at the top of a building or on a purpose-built aerial platform, is an important factor when considering the environmental impact of a base station. Since each of a number of cellular mobile radio systems which may operate in a single territory requires a large number of base stations spaced throughout the territory (some placed in environmentally sensitive rural or urban areas) it is beneficial to provide an antenna arrangement capable of providing a higher uplink diversity receiving gain whilst maintaining an environmentally acceptable aerial size.
The present invention allows a base station antenna configuration to be considerably smaller than that of the four-branch space-diversity aerial. It can also be used to reduce the mechanical complexity of the physical antenna configuration.